Polyamide (nylon) thermoplastic resins offer an excellent balance of processability and performance properties and therefore are used widely. Of the many types of nylon available, the most common are Polyamide-6 and Polyamide-6,6. However, some end-use applications for these resins require improved impact performance at ambient or low temperatures. For these applications, polyamide is usually toughened by impact modifiers also called “tougheners”. Polymer toughness, in the form of impact strength or resistance, is a measure of the ability of a material or processed article to withstand the application of a sudden load without “failure”.
Illustrative polyamide impact modifiers that can be used for this purpose are grafted-maleic anhydride elastomers or terpolymers where the level of maleic anhydride is usually less than 5% and typically in the range of 0.8 to 2%. Examples from the class of maleic-anhydride grafted elastomers are thermoplastic olefins (TPO) also called reactor TPOs, rubber copolymers produced in a reactor from ethylene and propylene (EPR), or rubber terpolymers of ethylene, propylene and diene-modifier (EPDM), plastomers of ethylene with an alpha-olefin, etc., all grafted with maleic anhydride off-line and random terpolymers of ethylene, acrylic ester and maleic anhydride with typical maleic anhydride contents in the range of 1-5%. In the case of EPDM, typically dienes currently used in the manufacture of EPDM rubbers are dicyclopentadiene (DCPD), ethylidene norbornene (ENB), and vinyl norbornene (VNB) used at a 1-12% level. Other examples include maleic-anhydride grafted olefinic thermoplastic elastomers (TPE) produced from copolymers of monomers like butadiene, isoprene, propylene, ethylene, butene and octene, which behave like elastomers in performance but process like thermoplastics. Styrenic type modifiers such as linear tri-block copolymer of styrene, ethylene and butylene with grafted maleic-anhydride groups are also sometimes used. Another example of an impact modifier is a terpolymer like an ethylene-acrylate ester-maleic anhydride terpolymer, where the acrylate ester is a methyl, propyl, butyl and other acrylate esters. It is well known to one skilled in the art that these elastomeric materials without the compatibilizing functional maleic anhydride group are not able to provide impact strength improvement to the compound, because in such a case the nylon and the elastomeric phase in the toughener have limited or no interaction and stress cannot be transferred from the rigid polyamide phase to the elastomeric phase which can withstand the impact energy. The use of these maleic-grafted elastomers to impart impact resistance to polyamides has been in commercial use for several decades and has been previously described in U.S. Pat. Nos. 4,174,358 and 4,594,386. Other impact modifiers used are the acrylic core-shell type such as the Paraloid® product line from The Dow Chemical Company. Grades that work well for polyamides are the polymers or copolymers with maleic anhydride pendant groups or with isocyanate groups. Occasionally ionomers such as Surlyn are also used as impact modifiers in polyamides but only for end use applications where low temperature impact is not required. Compatibilizers (i.e. polymers or copolymers that, when added to an immiscible polymer blend, modify the blend's interfacial character and stabilizes its morphology) can be added with elastomeric materials that do not include compatibilizing functional groups.
Addition of a “good” toughener results in “no break” of the test sample during the notched Izod impact strength with typical values greater than 800 J/m (15 ft-lb/in.) at room temperature when used at levels between 15-25 weight %. The impact strength measured by the notched Izod method or by Charpy impact method depends on testing temperature. Use of a nylon modifier may provide high values of toughness at room temperature, yet only achieve a lower level of toughness below or at −30° C. Typically, the stiffness, thermal properties like softening point and heat deflection temperature (HDT) of a toughened polyamide decrease as more toughener is added with significant decreases in properties, e.g. flex modulus and tensile strength. Also the more elastomeric or its lower the glass transition temperature (Tg) the underlying impact modifier the better is its impact strength at low temperature. The same trend in physical properties is observed when the polyamide being toughened is reinforced with reinforcements like glass fibers, wollastonite or talc mineral fillers and/or flame retardants to form polyamide composites.
It is commonly accepted in the plastics compounding industry that when these impact modifiers are compounded into polyamides, there are negative effects on other properties such as tensile strength, tensile modulus, flexural modulus and strength as well as thermal properties such as heat deflection temperature (HDT) and softening point. However from an end user perspective, there remains a need for polyamide compounds with material compositions which mitigate these negative effects. There is a need in the marketplace for compositions which yield high values of impact performance of the polyamide compound without causing significant decrease in its other mechanical properties (i.e. retaining or even improving the impact properties of parts produced from such modified polyamide compounds and molded or extruded articles produced from those compositions).